Note : Consider the following four compounds for answering questions 19 and 20.
I. `CH_3 - CH_2 - CH_2 - overset(overset(O)||)C - H`
II. `CH_3 - CH_2 - CH_2 - overset(overset(O)||) C - CH_3`
III. `CH_3 - CH_2 - underset(underset(O)||) C - CH_2 - CH_3`
IV. `CH_3 - underset(underset(CH_3)|)CH - CH_2 - underset(underset(O)||)C - H`
Which of the following pairs are position isomers ?

Options

A. I and II
B. II and III
C. II and IV
D. III and IV

Explanation

To determine which pairs of the given compounds are positional isomers, we need to analyze the structure and functional groups of each compound. Positional isomers are compounds that have the same molecular formula and functional groups but differ in the position of the functional group within the molecule.

### Step-by-Step Solution:

1. **Identify the Compounds:**
- Compound I: `CH3 - CH2 - CH2 - C(=O) - H` (Aldehyde)
- Compound II: `CH3 - CH2 - CH2 - C(=O) - CH3` (Ketone)
- Compound III: `CH3 - CH2 - C(=O) - CH2 - CH3` (Ketone)
- Compound IV: `CH3 - C(CH3) - CH2 - C(=O) - H` (Aldehyde)

2. **Identify Functional Groups:**
- Compounds I and IV both contain an aldehyde group (−CHO).
- Compounds II and III both contain a ketone group (−C(=O)−).

3. **Count the Carbon Atoms:**
- Compound I has 4 carbon atoms.
- Compound II has 5 carbon atoms.
- Compound III has 5 carbon atoms.
- Compound IV has 5 carbon atoms.

4. **Check for Positional Isomers:**
- **Comparing Compound I and IV:**
- Different number of carbon atoms (4 vs. 5). Not isomers.
- **Comparing Compound II and III:**
- Both have 5 carbon atoms and a ketone group.
- In Compound II, the ketone is at the 2nd position.
- In Compound III, the ketone is at the 3rd position.
- Since they have the same functional group but differ in the position of the ketone, they are positional isomers.

5. **Conclusion:**
- The only pair that are positional isomers are Compound II and Compound III.

### Final Answer:
The pairs that are positional isomers are **Compound II and Compound III**.